1. Field of Invention
This invention relates to alignment systems. Specifically, the present invention relates to systems and methods for calibrating reference sources and accompanying reference source steering mechanisms in systems for aligning sensors or transmitters to desired optical paths.
2. Description of the Related Art
Alignment systems are employed in various demanding applications including imaging, chemical analysis, and military targeting, surveillance, and reconnaissance systems. Such applications require alignment systems that can accurately and efficiently align components, such as optics, along a desired line-of-sight. These applications often require precise alignment of multiple constituent sensor components to ensure accurate handover of sensing function from one sensor to another or to facilitate multi-sensor data integration or fusion.
Precise system component alignment is particularly important in multi-spectral electro-optical systems employing multiple sensors sharing a common aperture. An exemplary electro-optical sensor suite includes a laser transceiver, a visible camera, and an infrared imager. The laser transceiver transmits a laser beam toward a scene. The scene reflects the beam, which is detected by the transceiver. The transceiver includes electronics and may include software to measure the round trip delay between transmission and reception of the beam and thereby determine the distance to a specific location within the scene, which may be a target.
The infrared imager detects thermal energy emanating from the scene. Electronics within the infrared imager convert received thermal energy into an image. Similarly, the visible camera receives visible-band electromagnetic energy reflected from the scene and generates a corresponding image. The infrared and visible images may be combined with laser range information to facilitate targeting or sensing. Generally, the center of the received reflected laser beam should coincide with the center or aimpoint of the infrared and visible images for accurate targeting.
Accurate and efficient alignment systems are particularly important in missile and aircraft sensor suites, where excess shock and vibration may exacerbate alignment problems. A sensor suite may include one or more reference sources rigidly attached and aligned to sensor suite components, such as detector arrays and lasers requiring alignment to a predetermined line-of-sight. In the exemplary sensor suite, the range finding laser requires alignment with a passive sensor, such as the visible or infrared sensor. A first reference source is rigidly aligned to the range finding laser and transmits a beam that is coincident with the transmitted laser beam. A beam splitter directs the first reference source beam onto the surface of a photodetector. A second reference source is rigidly aligned to the optical axis of the passive sensor and transmits a second reference beam coincident with the optical axis of the passive sensor. The second reference beam is also directed to the photodetector via the beam splitter. The laser is aligned with the passive sensor by aligning the dot caused by the first reference source with the dot caused by the second reference source on the surface of the photodetector. The reference dots are often aligned to the center or null position on the photodetector.
One or more steering mirrors are often placed in the optical path of one or more of the sensor suite components, such as detectors or lasers, requiring alignment. The mirrors are controlled by alignment loops, which adjust the angle of each mirror to align the lines-of-sight of sensor suite components.
To accurately align sensor suite components, a command sent to an alignment mirror to move the mirror a predetermined amount in a predetermined direction must accurately move the mirror by that predetermined amount and in that predetermined direction. In addition, the reference spot on the surface of the photodetector should move by the desired amount in relation to the angular movement of the mirror. To achieve this, alignment systems often require so-called reference source calibration.
The movement of a reference spot on the surface of a photodetector is proportional to the angular movement of the steering mirror. The proportionality constant is a scaling factor that includes photodetector sensitivity, optical reference beam intensity, and reference beam diameter, which are unique to each photodetector and reference source. Consequently, each combination of photodetector and reference source has a unique scaling factor describing the relationship between reference spot motion on the surface of the detector and angular movement of the steering mirror. Scaling factor accuracy requirements are higher than manufacturing build-up tolerances. Consequently, this scaling factor is conventionally determined through reference source calibration after assembly of the accompanying sensor suite.
The measured or commanded reference source positions (spots) on the photodetector must be accurate and must be calibrated for optimal component alignment, which is especially important in applications requiring off-center line-of-sight positions (peripheral positions) which are displaced relative to the center of the photodetector. In a non-calibrated system, a command to move a reference spot by certain angle may cause the reference spot to move by a different angle. In a calibrated system, calibration scale factors are applied to cancel this difference so that the reference spots move as commanded.
Conventionally, reference sources are calibrated manually. An alignment system is tested during manufacture or in the laboratory with specialized equipment to determine the correct calibration scale factors to apply to the position commands for steering the mirrors to achieve desired movement characteristics of a given reference spot on the surface of a photodetector. However, manual calibration is often undesirably time-consuming and expensive. Manual calibration in the field is particularly problematic.
Shock and vibration during missile or aircraft maneuvers may cause sensor suite components to shift or malfunction. Certain sensors and reference sources may require replacement. When components are interchanged or replaced, the reference source calibration scale factors must be adjusted manually via factory calibration equipment. Conventionally, this requires that the sensor suite be shipped back to the factory, which is inefficient, expensive, and time-consuming.
Hence, a need exists in the art for an efficient and accurate system for calibrating reference sources that does not require scarcely available factory equipment. There exists a further need for an efficient sensor component alignment system that incorporates the efficient system for calibrating reference sources.
The need in the art is addressed by the system for calibrating an apparatus for aligning components relative to a desired path of the present invention. In the illustrative embodiment, the inventive system is adapted for use with a sensor suite. The system includes first mechanism for generating a command designed to move a line-of-sight of one of the components to a first position, the line-of-sight moving to a second position in response thereto. A second mechanism automatically compensates for a variation between the first position and the second position via a scale factor.
In a more specific embodiment, the system further includes a third mechanism for adjusting the command via the scale factor so that the second position matches the first position. The line-of-sight is coincident with a first reference beam. The command corresponds to a mirror drive signal that controls a steering mirror positioned to selectively alter the line-of-sight.
The components are sensor system components and include one or more electromagnetic energy transmitters, receivers, and/or sensors. The second mechanism includes a photodetector. The steering mirror is responsive to the command and is positioned to direct the first reference beam onto the photodetector, thereby creating a reference spot corresponding to the reference beam on the surface of the photodetector.
The second mechanism includes a processor that communicates with the steering mirror and receives output from the photodetector. The processor runs software for generating the command and measuring the first position and the second position of the reference beam by measuring corresponding positions of the reference spot on the surface of the photodetector. The software receives input from an image tracker that communicates with an imaging sensor. The imaging sensor is rigidly aligned to the reference source and aimed at a calibration target. The input received by the processor from the image tracker represents a change in angular position of an image of the target in response to the command.
The software includes a module that adjusts the scale factor based on the variation in angular position of the image. The variation corresponds to a difference in angular position between the first position and second position. The module for adjusting scale factor implements the following equation:
(K Cal Error Comp)t=(K Cal Error Comp)txe2x88x921(xcex94X,Y t/xcex8CDP),
where (K Cal Error Comp)t is the scale factor at time t; (K Cal Error Comp)txe2x88x921 is the previous scale factor at time txe2x88x921; xcex94X,Y t/xcex8CDP is a scale factor correction term based on the variation in angular position, where xcex94X,Y t is the difference between a null position and a resulting position of a line-of-sight of a sensor corresponding to the second position; and xcex8CDP represents the command corresponding to the first position.
In an illustrative embodiment, the components to be aligned to the desired line-of-sight include an active sensor employing a laser beam or other beam of electromagnetic energy. The components also include a first reference source for providing the first reference beam and a second reference source for providing a second reference beam. The first reference beam is aligned to a first component, and the second reference aligned to a second component. The steering mirror is a common steering mirror that is positioned in the path of both the first reference beam and the second reference beam. The common steering mirror directs the first reference beam and the second reference beam onto the surface of the photodetector.
The software includes a module that determines calibration scale factors for the first reference source to yield a calibrated reference source in response thereto. The module for determining calibration scale factors employs the calibrated reference source to determine calibration scale factors associated with the second reference source. The second reference source has an additional steering mirror in its line-of-sight.
The novel design of the present invention is facilitated by the second mechanism, which automatically determines the scale factor that enables accurate auto-alignment commands that match actual changes in the lines-of-sights of components to be aligned. Automatically determining required calibration scale factors obviates tedious manual calibration. Consequently, certain sensors may be replaced in the field, which saves costly downtime.